In the making of multiple layer flexible plastic-type films, after the film is produced, it is wound up in roll form for storage or shipment. It is normal to use, as one of the outer layers of the film, a tacky heat sealable material, such as ethylene vinyl acetate. The other outer layer of the film, on its other surface, may be a non-sealant layer composed of a non-tacky type polymer.
If the tacky material is used as the entire composition of the one outer layer, the tackiness may function as an adhesive in the roll, such that the tacky sealant layer sticks to the non-sealant layer.
To avoid the above problem of the tacky sealant layer sticking to the non-sealant layer, it has been normal practice to incorporate a slip additive into the tacky layer to reduce its tackiness. It is known that the slip additive gradually migrates to the outer surface of the tacky layer. When the film has been in roll form and is subsequently unrolled, testing reveals that the coefficient of friction (COF) of the sealant layer has increased since the roll was made, and the COF of the non-sealant layer has descreased.
These films are commonly used with packaging machinery which is sensitive to the COF of both surfaces of the packaging material. Desirably, the COF of the non-sealant layer is about 0.4 to 0.5, and the COF of the sealant layer is 0.2 to 0.3. Since the packaging equipment is sensitive to the COF of the two surfaces of the packaging material, it is highly desirable that the COF of the packaging material be predictable, and substantially constant with time.
It is an object of this invention to provide a multiple layer, heat sealable, flexible packaging material, having a slip additive in the heat seal layer, which material can be stored in roll form, and in which the coefficients of friction of the two surfaces of the film are substantially constant with time.